1. Field of the Invention
The invention relates to an improved method of preparing the crystalline aluminosilicate zeolite, ZSM-20, by a seeding method which shortens the time required to crystallize the product.
2. Description of the Prior Art
Certain zeolite materials are ordered, have porous crystalline aluminosilicate tetrahedrally coordinated frameworks with specific crystalline structures and have a large number of small cavities interconnected by channels. These cavities and channels are uniform in size and have dimensions that absorb molecules of particular dimensions while rejecting others. As a result, these materials have come to be known as "molecular sieves" and are used in a variety of ways to take advantage of this property.
Molecular sieves include a wide variety of crystalline aluminosilicates having rigid tetrahedral frameworks of silicon and aluminum, crossed-linked by shared oxygen atoms. The electronic valence of the aluminum in the tetrahedra framework may be balanced by adding a cation to the framework. For example, an alkali metal or alkaline earth metal cation may be wholly or partially exchanged by a different cation using conventional ion exchange techniques. As a result, the properties of the aluminosilicate will vary. Of particular interest and importance are large pore 12-ring molecular sieves, such as ZSM-20. U.S. Pat. No. 3,972,983 describes the original method for preparing ZSM-20, which has the general formula: EQU (0.3-0.6)R.sub.2 O:(0.4-0.7)M.sub.2 O:Al.sub.2 O.sub.3 :(x)SiO.sub.2
where R is a tetraethylammonium cation, M is an alkali metal cation and x is at least 7. The preparation involves forming a reaction mixture comprised of an alkali metal oxide source, preferably sodium oxide, tetraethylammonium oxide, an aluminum oxide, tetramethylorthosilicate (TMOS) source of silicon oxide and water. The reaction mixture, which resembles a gel, is then heated to 50.degree. C. for 1 to 7 weeks and heated again between 90.degree. C. to 120.degree. C. for 2 weeks to 1 month. Digestion of the gel particles occurs until a crystal product forms. The crystals are separated from the reaction mixture by cooling the entire mixture to room temperature, filtering out the crystals and washing them. Finally, the crystals are dried at 100.degree. C., for 1 to 24 hours.
An improved process for preparing ZSM-20 is described in European Patent 0012572, where the TMOS is replaced with tetraethylorthosilicate (TEOS). This method produces a residual ethyl alcohol by-product that is believed to be less detrimental to product quality than the residual methyl alcohol, produced in U.S. Pat. No. 3,972,983 when TMOS is used.